Detail of GB/T 26641-2021

Introduction of GB/T 26641-2021

1 Scope This document specifies the terminology and definitions for non-destructive testing (NDT) by the metal magnetic memory (MMM) technique and the general technical requirements for the application of the method. The NDT technique specified in this document has the following purposes: --- to determine the non-homogeneity of the magnetomechanical state of a ferromagnetic object, to detect the degree of stress concentration caused by defects and the boundaries of metal microstructural inhomogeneities; --- to identify locations with surface magnetic distortions for further microstructural analysis and/or non-destructive testing and evaluation; --- early damage diagnosis of the inspected object and evaluation of its structural life; --- Rapid classification of new and used inspection objects by magnetic heterogeneity for further testing; --- Magnetic memory in combination with other NDT methods or techniques (ultrasonic testing, X-ray testing, etc.) allows rapid detection of the most likely locations of defects, thus increasing the efficiency of NDT; --- for quality control of all types of welded joints and their execution (including friction and spot welding). For specific applications see ISO 24497-2. 2 Normative references The contents of the following documents constitute essential provisions of this document by means of normative references in the text. Where a reference is dated, only the version corresponding to that date applies to this document; where a reference is not dated, the latest version (including all amendment sheets) applies to this document. ISO 9712 Qualification and certification of non-destructive testing personnel Note: GB/T 9445-2015 Qualification and certification of NDT personnel (ISO 9712:2012, IDT) ISO/T S18173 General terms and definitions for non-destructive testing 3 Terminology and definitions The terms defined in ISO/T S18173 and the following terms and definitions apply to this document. The terminology database used in standardisation work maintained by ISO and IEC is available at the following URLs: 3.1 Magnetic memory of metals The magnetic state of a ferromagnetic object after it has undergone magnetic field changes and the cumulative effect of magneto-mechanical effects. Note: For a given magnetic field (e.g. geomagnetic field), a ferromagnetic object formed during its manufacture or operation, the residual magnetisation strength is changed due to various environmental factors affecting the distribution of magnetic domains [35] (e.g. temperature, mechanical loads [6] [10] [17] or changes in the microstructure of the material). 3.2 Surface magnetic fields The magnetic field leaving or entering the surface of a part and unintentionally magnetising the part. Note 1: A ferromagnetic material generates a magnetic field in its own volume and in the surrounding space. The field generated by the magnetisation distribution of the material itself is called the surface magnetic field or the degenerate magnetic field within it. The degenerate magnetic field and the surface magnetic field are geometrically related and occur when the magnetisation intensity is not uniform or has a component normal to the external or internal surface [46]. High local variations in the surface magnetic field, similar to flux leakage, can indicate inhomogeneities in material properties. Note 2: Other terms used in the literature are, for example, spontaneous leakage magnetic field, residual magnetic field, surface magnetic field, leakage magnetic field, magnetic field density or surface field. When used in non-destructive testing, surface magnetic field is the recommended term for passive magnetic field measurements, while leakage defines the magnetic flux that is amplified by external magnetisation prior to or during testing. 3.3 Magnetic memory testing of metals A non-destructive testing technique by measuring and analysing the surface magnetic field [3.2] distribution of the inspected object [IOs] without active magnetisation. Note: The magnetic field sensitive probe is used to measure the surface magnetic field distribution. 3.4 Surface magnetic field vector HSF,i The component of the surface magnetic field in direction i (i=x,y,z) of the object under inspection, determined using passive magnetic field sensing. 3.5 Surface magnetic field indication strayfieldindication;SFI Deviation of the SF (surface magnetic field) caused by high mechanical stress/strain gradients [6][10][17][47]. Note 1: SFI also forms at locations with localised changes in magnetic permeability, which can be caused by concentrations of defects (e.g. caused by cracks, pitting), strongly heterogeneous boundaries in the metal tissue, impurities, abrupt geometrical changes [24][25][57][60], internal and external surfaces [46], separation from the object under examination, irreversible deformation (high dislocation density) and changes in chemical composition (e.g, deposition or leaching), etc. Note 2: Surface magnetic field indications are not necessarily indicative of defects and need to be interpreted to determine their relevance Surface magnetic field indications replace stress concentration zones [use SCZ only where mechanical stresses are concentrated (e.g. sharp corners, crack tips)], see Appendix A. 4 General requirements 4.1 Magnetic memory techniques are based on the measurement and analysis of the magnetic field distribution on the surface of ferromagnetic objects. The magnetisation intensity reflects the microstructure, manufacturing process and working load of ferromagnetic metal components (including welded joints). The inspection should use the surface magnetic field generated by the residual magnetic field formed by the ambient magnetic field during the manufacturing process and during the service life of the object under inspection. 4.2 Magnetic memory inspection techniques can detect surface magnetic field indications and provide recommendations for alternative non-destructive testing methods for welded joints in ships, pipelines, equipment (e.g. steam boilers, turbines, heat exchangers, rails) and structural components. The inspection of welded joints shall be carried out in accordance with ISO 24497-2. Note: The magnetic indication of the surface of the inspected object is formed by the manufacturing technique (fusion, forging, rolling, turning, press forming, heat treatment, etc.). 4.3 Under certain conditions, especially in the presence of ferromagnetic phases (e.g. sub-stable austenitic steels, oxidation, coatings), magnetic memory inspection techniques can be used for the inspection of non-magnetic inspected objects. Note: Sub-stable austenitic steels can be inspected if their organisation is sensitive to γ-α phase changes. The evaluation of the surface magnetic field is limited to the ferromagnetic phase. 4.4 When performing magnetic memory testing, the temperature should be within the normal and safe working range of the operator (NDT personnel). 5 Test objects 5.1 The equipment and structures to be tested should be subjected to magnetic memory testing both in service (under load) and in maintenance (after working load has been removed). If possible, it is advisable to obtain the initial magnetic state of the object to be inspected. 5.2 The surface of the object to be inspected does not need to be treated. It is advisable to reduce the lifting distance of the probe by removing the insulating layer in order to improve the reliability of the detection and to avoid the indication of the surface magnetic field due to the insulating layer. In exceptional cases, non-magnetic insulation may be permitted during the inspection. All permitted insulation layers should be experimentally verified and the results should be attached to the inspection report. 5.3 Limitations on the application of MMM testing include the following: --- Demagnetisation and magnetisation of the object under test; --- External (electromagnetic) magnetic fields in the vicinity of the inspected object, in the vicinity of the inspected area; --- temperature variations that can affect the results (e.g. Curie temperature); --- the distance from the probe to the surface of the object to be inspected (lifting off) and its change during the measurement. 5.4 It is advisable to take into account changes in thermal remanence due to severe temperature changes in the object to be examined when processing the results. 6 Detection equipment 6.1 A sufficiently sensitive magnetic probe should be used to measure the surface magnetic field near the surface area of the object to be inspected, either using a magnetometer or a gradiometer configured with a magnetically sensitive probe (e.g. fluxgate probe). 6.2 The test instrument should have a display of the test parameters, digital data acquisition and storage, and position-coded movement of the probe. The external computer interface should allow for external data storage, retrieval and display of results. It is desirable that the instrument is available at the same time as the external evaluation software. 6.3 The type and size of the probe is determined by the specific testing requirements. The test instrument should have at least two measurement channels, one for the measurement of the surface magnetic field of the object to be inspected and the other to compensate for the influence of the external magnetic field He. The probe type and settings (e.g. gradiometer/magnetometer) should be documented in the test report. 6.4 The probe should be controlled by a sweeper and the position encoder should give the actual probe position during the sweep. For objects where it is difficult to use the sweeper, the probe should acquire data in real time. 6.5 The following factors influence the measurement of surface magnetic fields: --- The probe is lifted away from the surface of the object to be inspected; --- The probe sampling rate when sweeping along the surface of the object; ---Probe sensitivity; ---Probe size; ---Probe sensitivity direction with respect to the subject; ---The orientation of the probe with respect to an external field source (e.g. the earth's magnetic field). 7 Preparation for testing 7.1 The test preparation procedure should include the following basic steps: --- analysis of the technical documentation of the inspected object, preparation of the testing progress chart (testing plan, preparation of the inspected object log files); --- Selection of probes and equipment; ---Preparation of written test procedures; --- Set-up and calibration of instruments and probes according to written operating instructions; --- The subject is divided into separate inspection areas and inspection units, which are marked in the subject log file. 8 Inspection 8.1 Normally the magnetisation intensity of the object to be inspected is unknown. The three Cartesian components of the surface magnetic field should be measured by a continuous or discrete sweep along the surface of the object being inspected. The probe should be in the same direction as the sweep, otherwise it should be recorded in the test report. The position of the object in relation to the external magnetic field should not be changed during the test. A dense grid of measurement lines should be planned on the surface of the object to be inspected. The location of the extreme HSF,i changes on the surface of the inspected object shall be determined and recorded. 9 Inspection report Appendix A (informative) Examples of methods for marking the magnetic field distribution on surfaces Bibliography

Contents of GB/T 26641-2021

1 Scope 2 Normative references 3 Terminology and definitions 4 General requirements 5 Test objects 6 Detection equipment 7 Preparation for testing 8 Inspection 9 Inspection report Bibliography